Re: [patch 2.6.22-rc2-git] spidev compiler warning gone

[David Brownell <david-b-yBeKhBN/0LDR7s880joybQ@public.gmane.org>]

> > > > Get rid of annoying GCC warning on 32-bit platforms.  The trick is to
> > > > add an extra cast using "ptrdiff_t" to convert the u64 to the correct
> > > > size integer, and only then casting it into a "void *" pointer.
> > > > 
> > > >	...
> > > > -			if (__copy_to_user((u8 __user *)u_tmp->rx_buf, buf,
> > > > +			if (__copy_to_user((u8 __user *)
> > > > +					(ptrdiff_t) u_tmp->rx_buf, buf,
> > > > 	...
> > > 
> > > This seems overly tricky.  The way we normally squish that warning is
> > > via a (long) cast.
> > 
> > Well, "ptrdiff_t" seems equivalent to "long" in common cases.
> > 
> > Presumably C99 guarantees pointer-to-long conversions work in
> > both directions?  It seemed there must necessarily be such a
> > guarantee for "ptrdiff_t", even if it weren't true for "long".
> > I didn't have a copy of a C spec to consult.  ;)
> > 
> 
> Thing is, the kernel does a cast of an integer-type to a pointer-type in a
> *lot* of places.  And the standard way in which we squish the warning is
> via a (long) cast.

Good to know, but I didn't notice any examples anywhere in the source
code of doing that.  I used the first solution I found, when I went
hunting for a fix.  It wasn't "long".  And it appears that "long" has
fewer portability guarantees...

Something in Documentation/* should talk about 64-bit cleanliness,
and maybe mention such issues.


> So it's a standard pattern and people understand it.  Whereas the ptrdiff_t
> thing will make readers go "wtf?".  Even if it deos happen to work.

At this point, "long" would surprise *me*!  I notice you didn't
answer my question about C specs and "long".  "ptrdiff_t" sure seems
to be a standard C data type designed specifically to solve that
class of 32/64/... portability problem.  "intptr_t" would seem to be
the most standard type, but I don't see it in kernel includes.

I notice that GCC docs say ptrdiff_t "will probably be one of the
standard integer types (short, int, long) but might be a nonstandard
type that exists only for this purpose".  So using "long" isn't as
portable, and isn't even guaranteed to work everywhere.

- Dave


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